The present invention relates to a passive optical network system, an optical line terminal, and optical network units. More particularly, it relates to a passive optical network system in which a plurality of subscriber connection devices share an optical transmission line.
In access communications networks whose subscribers are connected to each other, significant progress has been also made toward the high-rate/broad-band implementation of the communications networks. In accompaniment with this trend, there have been projects for the introduction of a Passive Optical Network system (which, hereinafter, will be referred to as “PON”). The PON is defined in such recommendations as the recommendation G984.3 made by the International Telecommunication Union Telecommunication Standardization Sector (which, hereinafter, will be referred to as “ITU-T”). The PON is a system in which an Optical Line Terminal (which, hereinafter, will be referred to as “OLT”) and Optical Network Units (which, hereinafter, will be referred to as “ONUs”) are connected to each other via a passive optical network including a main optical fiber, an optical splitter, and a plurality of branch-line optical fibers. Here, the OLT corresponds to a master station connected to a higher-order communications network, and the ONUs correspond to slave stations containing terminals (i.e., PCs or telephones) of a plurality of subscribers. Concretely, the mode of communications performed in the PON is as follows: First, signals from the terminals (i.e., PCs) connected to the respective ONUs are converted into optical signals. Next, these optical signals are optically (time-division) multiplexed by the optical splitter from the branch-line optical fibers into the main optical fiber, then being transmitted to the OLT. Moreover, the OLT applies a communications processing to these (time-division) multiplexed signals from the respective ONUs. Finally, the OLT transmits the communications-processed signals to the higher-order communications network, or to another ONU connected to the OLT.
The introduction of the PON had started with the system which addresses 64-Kbit/s low-rate signals. Moreover, the introduction of the following PONs is underway at present: B-PON (: Broadband PON) which transmits/receives fixed-length ATM cells at about 600 Mbit/s, E-PON (: Ethernet PON) which transmits/receives variable-length packets of Ethernet (: registered trademark, indication of the registered trademark will be omitted hereinafter) at about 1 Gbit/s at the maximum, and G-PON (: Gigabit-capable PON) which addresses about 2.4-Gbit/s higher-rate signals. Furthermore, in the near future, there will be a request for the implementation of high-rate PONs which are capable of addressing 10-Gbit/s to 40-Gbit/s signals. In addition, as the method for implementing these high-rate PONs, consideration is nowadays given to the utilization of TDM (: Time-Division Multiplexing) for time-division multiplexing a plurality of signals in much the same way as in the already-existing PONs. Incidentally, the configuration of the already-existing PONs is as follows: Different wavelengths are employed and used between an upstream (i.e., from each ONU to the OLT) signal and a downstream (i.e., from the OLT to each ONU) signal, and a communication time is assigned to each ONU in the communications between the OLT and each ONU. Concretely, this configuration is that burst-like variable-length signals (i.e., burst signals) are assigned which can make it easier to address various types of signals (such as voice, picture, and data).
In each of the above-described PONs, the ONUs are set up in subscribers' homes which are positioned at various places. Accordingly, the distances ranging from the OLT to the respective ONUs are different from each other. Namely, there occurs a variation in the lengths (i.e., transfer distances) of the optical fibers constituted by the main optical fiber and the branch-line optical fibers from the OLT to the respective ONUs. Consequently, there occurs a variation in the transfer delays (delay amounts) between the OLT and the respective ONUs. As a result, there exists a possibility that, even if the respective ONUs transmit the optical signals with different timings set thereto, the optical signals from the respective ONUs will collide/interfere with each other on the main optical fiber. In view of this situation, in each PON, after the distance measurements are carried out between the OLT and the respective ONUs, the delays of the signal outputs from the respective ONUs are adjusted so that the signal outputs from the respective ONUs will not collide with each other. This adjustment is made using, e.g., the ranging technology defined by the ITU-T recommendation G984.3. Also, based on transmission requests from the respective ONUs, the OLT determines the bandwidths of signals for granting transmission permissions to the respective ONUs. This determination is made using dynamic bandwidth assignment (which, hereinafter, will be referred to as “DBA”) technology. Then, the OLT specifies the signal transmission timings to the respective ONUs so that the optical signals from the respective ONUs will not collide/interfere with each other on the main optical fiber. This specification is performed such that the delay amounts measured with the ranging are also taken into consideration. Namely, each PON is configured so that the operation of the communications therein will be performed in a state where the timings of the signals to be transmitted/received between the OLT and the respective ONUs are managed inside the system.
In the above-described G-PON, each of the signals transmitted from the respective ONUs to the OLT is configured as follows: Namely, interference-preventing guard time formed of 12 bytes at the maximum, preamble utilized for determination of signal-identifying threshold value for an in-OLT receiver and clock extraction, burst overhead byte referred to as “delimiter” for identifying separation of a received signal, and a control signal for the G-PON (which is also referred to as “overhead or header” in some cases) are added to the front end of the data (which is also referred to as “payload” in some cases). This configuration is implemented so that the signals from the respective ONUs which are (time-division) multiplexed onto the main optical fiber can be processed in a manner of being identified with each other by the OLT. Incidentally, each data is variable-length burst data, and a header which is referred to as “GEM (: G-PON Encapsulation Method) header” for processing the variable-length data is added to the front end of each data.
Meanwhile, each of the signals transmitted from the OLT to the respective ONUs is configured as follows: Namely, frame synchronization pattern for identifying the front end, PLOAM area for transmitting monitor/maintenance/control information, and overhead which is referred to as “grant instruction area” (also referred to as “header” in some cases) for instructing the signal transmission timings for the respective ONUs are added to the front end of each of the data which are (time-division) multiplexed in a manner of being addressed to the respective ONUs. This configuration is implemented so that the signals transmitted from the OLT can be processed in a manner of being identified with each other by the respective ONUs. Incidentally, similarly to each of the signals from the respective ONUs, the GEM header for processing the variable-length data is added to the front end of each of the multiplexed data addressed to the respective ONUs. Using the grant instruction area, the OLT specifies the upstream transmission permission timings (i.e., transmission Start and End) for the respective ONUs. This specification is performed with respect to the respective ONUs in a byte unit. These transmission permission timings are referred to as “grants”. Furthermore, when the respective ONUs transmit the plural pieces of OLT-addressed data in accordance with the transmission permission timings, these plural pieces of data are optically (time-division) multiplexed onto the main optical fiber, then being received by the OLT.